Environmental and technical characteristics of conductive adhesives versus soldering

3.Test cases with conductive adhesives

3.1 Case descriptions
3.2 Assessment of the test cases

The goal of the test cases was to reveal the possibilities and limitations of using electrically conductive adhesive as a replacement for solder. The work was performed in collaboration with four Danish electronic manufacturers: Danfoss A/S, Grundfos A/S, Bent Hede Elektronik A/S, and Mekoprint A/S. Each manufacturer performed a test case with electrical conductive adhesive. Four different substrates and substrate platings were used in the cases

1. FR-4 substrates with gold over nickel plating
2. Ceramic substrates with thickfilm silver conductors
3. 3D Polycarbonate substrate with depressed copper conductors for chemical nickel and gold plating
4. Polyester substrate with polymer silver conductors.

All mounted components had tin-lead plated terminals. In order to establish a reference the ceramic and FR-4 substrates were also tested with solder. This was not possible for the “low temperature” polycarbonate and polyester substrates since they cannot withstand the high soldering temperatures.

Parallel to the industrial cases DELTA evaluated 13 conductive adhesives on four different substrate platings and with two different component terminal finishes. These results are presented in chapter 8.

The overall result from the cases is that conductive adhesive is not considered to be a general replacement for solder, but conductive adhesives will be used especially on low temperature substrates and possibly where heat transfer is high, i.e. for shielding applications, ceramic substrates, and possibly for low current applications. If conductive adhesives should more generally replace solder a further development of the conductive adhesives will be necessary.

3.1 Case descriptions

The four cases are described in the below table by means of application, motivation, substrates, components, adhesives and various process parameters.

Look here

3.2 Assessment of the test cases

The results from the test cases were:

	electrical conductive adhesive shall be applied on noble metal surfaces, i.e. tin-lead as a substrate or component terminal finish is not recommended, due to oxidation.
	redesign of the footprints on the substrates may be necessary.
	reduction of the openings in the stencil is necessary to reduce the applied amount of electrical conductive adhesive compared to solder.
	the existing production equipment can be used i.e. the stencil printer or the dispenser, the component mounting automate and the furnace. A furnace for adhesive curing is recommended, especially if both adhesive and solder shall be used in the production.
	process optimisation with electrical conductive adhesive is more difficult than with solder, especially due to no self alignment and no contraction of the adhesive.
	curing shall be performed according to manufacturers specification, i.e. the temperature shall be controlled.
	interconnection resistance is higher with electrical conductive adhesives than with solder.
	the adhesion between component/adhesive/substrate seems to be lower for electrical conductive adhesives than for solder, this may not be correct for some SMD (ceramic chip resistors and capacitors) components mounted to thickfilm silver conductors on ceramic substrates. Depending on the electrical conductive adhesives adhesion strength and the component, components shall be mechanical fixed.
	repair may be a problem, as hand mounting of electrical conductive adhesives may give an increased interconnection resistance, and revealed silver flakes may tend to migrate.
	repair is estimated to be much more costly with electrical conductive adhesives than with solder.
	protection against uncured epoxy and epoxy vapour is required. Personal shall be trained to handle epoxy.
	for low temperature substrates there is only one alternative, which is low melting solder containing Indium.
	the price for electrical conductive adhesives is about 5 times higher than for solder.